The invention relates to the field of thermal turbomachines, in particular to a gas turbine, and to a method for controlling combustion in a gas turbine, in accordance with the preambles of patent claims 1 and 5.
A gas turbine of this type is known, for example, from EP 1 065 346 A1. In this turbine, temperature sensors are arranged in guide vanes belonging to a first row of guide vanes of a gas turbine. In each case one burner which is assigned to the temperature sensor is controlled on the basis of measured temperature values from in each case one temperature sensor. A temperature sensor is assigned, in an annular combustion chamber, one or more burners which lie opposite the sensor in a circumferential segment of the gas turbine and therefore influence its temperature most directly. The control evens out a temperature distribution over a circumference of the combustion chamber outlet, so that a rotor blade of the turbine is exposed to a temperature which is as uniform as possible during a revolution.
The temperature distribution of the combustion gases should be made uniform both in the radial direction and along the circumference, i.e. in the azimuthal direction, in order to minimize thermal loads on a first row of turbine guide vanes. Hot gas which originates from a flame is at a different temperature from gas which originates from areas between the flames or is added as cooling air. In conventional combustion-chamber designs, the maximum possible turbulence is imposed, resulting in mixing of the gas flows and to some extent the temperatures being made more uniform. However, this has the drawback that energy is required to produce this turbulence, which leads to a greater pressure drop in the combustion chamber and therefore to worse overall efficiency of the machine. Moreover, the higher degree of turbulence lengthens the residence time of the gas molecules in the hot area of the combustion chamber, with the result that the thermal formation of nitrogen oxides in accordance with the Zeldovic mechanism is increased.
An additional problem with modern gas turbine combustion chambers is the occurrence of combustion fluctuations which, as a result of fluctuating liberation of heat, generate pressure pulses, which in turn impose a high mechanical load on the materials of the combustion chamber and turbine. In recent developments, this load has been combated by active control of the combustion stability, as described, for example, in K. C. Schadow, E. Gutmark and K. J. Wilson xe2x80x9cActive combustion control in a coaxial dump combustorxe2x80x9d Combust. Sci. Technol. 81, 285 (1992) and in C. O. Paschereit, E. Gutmark and W. Weisenstein xe2x80x9cControl of thermoacoustic instabilities and emissions in an industrial type gas turbine combustorxe2x80x9d 27th International Symposium on Combustion, The Combustion Institute, Aug. 2-7 1998. However, one of the significant problems with practical use of the developments described in those articles is the determination of the nature of combustion fluctuations which occur and their causes by sensor means. Even if the causes are by now very well known from tests and are documented, for example in C. O. Paschereit, E. Gutmark and W. Weisensteinxe2x80x9cCoherent structures in swirling flows and their role in acoustic combustion controlxe2x80x9d Physics of Fluids 11, No. 9, September 1999, linking actual measurements with suitable countermeasures in some cases causes considerable difficulties, on account of the current deficiencies of sensor devices, when taking suitable countermeasures, particularly with azimuthal fluctuation modes.
Therefore, it is an object of the invention to provide a gas turbine and a method for controlling combustion in a gas turbine of the type described in the introduction which eliminates the above drawbacks.
This object is achieved by a gas turbine having the features of patent claim 1 and a method for controlling combustion in a gas turbine having the features of patent claim 5.
Therefore, as temperature sensors, the gas turbine according to the invention has spectrometers, which are designed to measure a combustion-gas temperature and are arranged so as to measure a gas temperature which prevails immediately in front of a first row of guide vanes.
The use of spectrometers allows precise, relatively high-frequency measurement of hot-gas temperatures. Moreover, drawbacks of thermocouples which are fitted in turbine blades, such as for example discrepancies or losses in quality of heat transfers between blades and thermocouples, are avoided.
In the method according to the invention for controlling combustion in a gas turbine, means for introducing fuel into a combustion chamber are controlled on the basis of measured temperature values, the temperature values being measured using spectrometers which are designed to measure a combustion-gas temperature and measure combustion-gas temperatures which prevail immediately in front of the first row of guide vanes.
In this way, it is possible to avoid excessive increases in temperature at the first row of guide vanes by controlling the supply of fuel. Combating of excessively high temperatures by means of deliberately produced turbulence is only required to a lesser extent, which leads to improved efficiency and to reduced nitrogen oxide emissions.
In a preferred embodiment of the invention, the temperature sensors are designed to measure a gas temperature of steam. For this purpose, in front of a photographic element which converts the recorded radiation into an electrical signal, it has a bandpass filter, which only transmits the radiation in frequency ranges which are only or predominantly characteristic of steam. In this way, it becomes possible to measure the temperature of the combustion gas directly, without contact and without disturbing the flow by having to fit additional components, on the basis of a typical exhaust-gas component. Steam has the advantage of having a very broad emission spectrum, which ensures a strong measurement signal and therefore a high measurement accuracy, and also of always being present in relatively large amounts when hydrocarbons are being burnt in the combustion gas. Similar measurements can also be carried out using carbon dioxide. In principle, it is also possible to measure the emissions of oxygen and nitrogen. All other possible exhaust-gas components generally count as air pollutants, the emission of which is to be reduced further by the use of modern techniques.
In a preferred variant of the method according to the invention, temperature values from a temperature sensor are used to determine an associated azimuthal gas-temperature profile in front of the first row of guide vanes. A deviation from an ideal, predetermined gas-temperature profile is recorded, and an associated means for introducing fuel is controlled on the basis of this deviation, in order to eliminate temperature peaks, or alternatively the need for maintenance is indicated.
In a further preferred variant of the method according to the invention, means for introducing fuel are controlled on the basis of temperature values from one or more temperature sensors, in order to compensate for axially symmetrical and/or azimuthal fluctuations in combustion. This is made possible by the measurement frequency of the sensors which are used according to the invention which is high compared to that of conventional sensors.
Further preferred embodiments will emerge from the dependent patent claims.